The present invention relates to a polymer-grafted fiber used to make composite materials. The invention relates in particular to a method for making such polymer-grafted natural fibers.
It is well known that composite materials are used in many applications on account of their excellent mechanical properties. The mechanical properties of composites are a function of the properties of the individual components.
Composites consist basically of two distinct phases: a first continuous phase (matrix) and a second dispersed phase. In many cases, the continuous phase (matrix) is a polymer. A second component is dispersed in the first continuous phase to make the second dispersed phase.
The dispersed phase in composites is usually designed to improve the mechanical properties of the matrix. The dispersed phase may have very different properties, form and dimensions, depending on the specific function it has to perform within the composite.
Many known composites are reinforced with fibers such as, for example, glass, carbon or synthetic polymer fibers.
Composites consisting of a polymer matrix with natural fibers as the reinforcing agent have only recently been introduced. The natural fibers most used for this purpose are of plant origin, and in particular, cellulose fibers.
The interest in materials reinforced with natural fibers can be ascribed to the following reasons:
natural fibers are obtained from renewable, low-cost sources;
natural fibers have very good mechanical properties;
natural fibers have a low specific weight compared to the synthetic or glass fibers usually used;
natural fibers are environmentally friendly since they are biodegradable and burn easily and can therefore be conveniently disposed of after use.
To obtain composites with good mechanical properties, the adhesion between the continuous phase (matrix) and the dispersed phase (fiber) must be optimized, since the mechanical load applied to the matrix must be effectively transferred to the fiber. In a composite, it is the fiber component that confers the good mechanical properties. The effectiveness of the adhesion at the interface between fiber and matrix depends largely on the surface properties of both components. Usually, the surface properties are correlated to parameters such as surface tension and polarity. The surface properties of the polymer matrices currently available on the market, used in the preparation of composites, are very different from the surface properties of natural fibers. However, to obtain good adhesion at the interface between fiber and matrix, the surface tension and polarity of the two components must be very similar.
The continuous phase (matrix) or the dispersed phase (fiber) can be chemically modified in numerous ways to suitably change the surface properties.
One way of chemically modifying the surface of a fiber is by grafting polymer chains onto it.
A fiber can be grafted with polymer chains which polymerize in situ or with ready-made polymer chains.
In the first type of grafting process, it is difficult to control the degree of polymerization and hence the length of the grafted chains.
The second process allows polymer chains of different types and preset length to be grafted.
U.S. Pat. No. 3,492,082 discloses a process for the preparation of polymer-grafted cellulose fibers.
In U.S. Pat. No. 3,492,082 the grafted cellulose fibers are prepared by converting the hydroxyl groups of the cellulose into hydroperoxide groups through the formation of an intermediate sulphonate ester group. The cellulose containing the hydrogen peroxide groups is subsequently reacted with a reactive monomer to yield the grafted cellulose material. The following monomers are used; styrene; butadiene; acrylonitrile; N-vinylpyrrolidone; acrylamide; and others. These monomers are capable of polymerizing with the hydroperoxide groups on the cellulose fiber in the presence of a free-radical initiator.
Hence, U.S. Pat. No. 3,492,082 describes a free-radical type, in situ polymerizing process. The free-radical polymerization produces on the fiber grafted chains with a very wide length distribution. The chain length of the polymer grafts thus varies considerably and cannot be predetermined. The kinetics of the free radical grafting reaction are difficult to control.
U.S. Pat. No. 4,857,588 discloses a process for the preparation of cellulose fibers with ready-made polymer grafts.
U.S. Pat. No. 4,857,588 describes a process in which the cellulose material is first treated with an aqueous sodium hydroxide solution. The cellulose material is then treated with sodium methoxide in methanol in order to convert the hydroxyl groups of cellulose into salified oxy groups with sodium in a quantity ranging from 0.25 to 33.3%.
Next, the resulting cellulose material is contacted with a ready-made organic compound having a chain with an electrophilic functional group at one end.
The main disadvantage of the fibers treated using the method described in U.S. Pat. No. 4,857,588 is that the fibers are subjected to a very drastic pre-treatment in an alkaline solution which alters their original structural, chemical and mechanical properties.
The treatment described by U.S. Pat. No. 4,857,588 (NaOH in 5N aqueous solution), modifies the crystal structure of the native cellulose fibers, as clearly shown by the X-ray diffraction spectrum. Indeed, following treatment as taught by U.S. Pat. No. 4,857,588 the spectrum shows the reflections typical of the crystal structure of regenerated cellulose known as cellulose II, as shown in FIG. 1B.
Therefore, one of the aims of the present invention is to provide natural fibers whose surface properties are modified in such a way as to obtain composite materials having improved mechanical properties.
Another aim of the present invention is to provide a process for the preparation of polymer-grafted natural fibers which leaves the mechanical properties of the fiber unchanged.
According to one aspect of it, as described in claim 1 hereof, the present invention provides a process for the preparation of polyether-grafted natural fibers.
This process keeps the mechanical properties of the natural fiber unchanged. Indeed, the chemical modifications do not affect the body of the fiber and do not alter its structure. Only the outer surface of the grafted natural fibers obtained using this process is modified.
Another advantage of the process according to the present invention is in that it reduces the hydrophilicity of the fiber so that the surface properties of the fibers become similar to those of the polymers used as matrices, thus improving the adhesion between the fiber and the polymer matrix. The dependent claims describe preferred embodiments of the process according to the present invention.
The invention also relates to a polyether-grafted natural fiber, as described in the corresponding independent claim, made preferably but not necessarily using the process according to the present invention.
The polyether-grafted natural fiber according to the present invention is highly compatible with the polymer matrices of numerous composite materials. Its surface properties are modified to improve the adhesion between it and the matrix of the composite material. Further, since only the surface structure of the natural fiber is modified, the body of the grafted fiber remains unchanged and maintains its mechanical resistance. The resulting composite material therefore has better mechanical properties than composite materials known up to now.
In a preferred embodiment, the grafted natural fiber is a cellulose fiber and the polyether has a general formula (I), as described below.
The invention also relates to a composite material comprising polyether-grafted natural fibers, as described in the corresponding independent claim, made preferably but not necessarily using the process according to the present invention.
In a preferred embodiment, the composite material comprises cellulose fibers grafted with a polyether having a general formula (I), as described below.
The present invention further relates to the use of polyether-grafted natural fibers made preferably but not necessarily using the process according to the present invention for the preparation of composite materials, as described in the corresponding independent claim.
In a preferred embodiment, the natural fibers used in the preparation of the composite materials are cellulose fibers prepared according to the process taught by the present invention.